Humidifiers are an important part of ventilator breathing circuits because breathing gas must be warm and humidified for optimal inhalation therapy. Humidification is particularly important for patients treated with ventilators for prolonged periods.
Available respiratory humidifiers can generally be placed in one of three groups according to their method of operation: nebulizing, bubbling and heated evaporation. Nebulizing humidifiers rely on the flow of pressurized breathing gas through an ejector-like element to generate an aerosol spray. With bubbling humidifiers, breathing gas is forced directly through a pool of liquid water. Heated-evaporation humidifiers employ a heated contact chamber, where breathing gas is passed over heated water to absorb water vapor.
A persistent, serious problem with all known humidifiers is condensation or "rainout" in the ventilator circuit downstream of the humidifier. As humidified breathing gas travels through tubing towards the patient, it is cooled by ambient air. If the breathing gas is overhumidified or if the room temperature drops, rainout occurs. Therefore, tubing cleanup and system readjustments become all too frequent steps in the maintenance of ventilator circuits.
As discussed in U.S. Pat. No. 5,558,084 to Daniell et al., the rainout phenomena is especially problematic in the home care environment, where precise control of room temperature may be unavailable.
Furthermore, condensation is considered contaminated waste and presents a disposal problem.
Efforts at overcoming the rainout problem are reflected in humidifiers with secondary heaters disposed downstream of the humidifier. For example, U.S. Pat. No. 4,621,632 to Bartels et al. is directed to a humidifier system that includes a coil of resistive heating wires within the breathing hose downstream of a main humidifier device. While those skilled in the art have long recognized that this multi-heater approach is overly complex and therefore expensive, a simpler yet successful approach to solving the rainout problem was previously unavailable.
Heated evaporation humidifiers are thought to suffer somewhat less from the rainout problem because rather than entrained droplets, they generate steam with molecular level distribution. Conventional heatedevaporation humidifiers present the related problem of overheating, however. Without careful monitoring, the temperature of heated water may rise above acceptable limits and cause breathing gas to become dangerously overheated. Equally dangerous are the steam surges produced when liquid water first contacts a dry heater bed following a disruption in water supply.
Another recognized disadvantage of heated evaporation humidifiers is over-dampening of ventilator pressure waves. For artificial respiration, the large gas volumes required in conventional humidifiers over dampen the pressure waves generated by the ventilator to inflate the patient's lungs.
The present invention addresses these and other problems with known humidifiers by providing a breathing gas humidifier with a relatively small gas volume and a flash-resistant evaporation module that can be placed relatively near the patient.